Initially, rigid contact lenses were constructed of poly(methyl methacrylate) and were impermeable to oxygen. As a result, wearers of such lenses would quickly complain of eye fatigue and eye strain due to a lack of oxygen to the cornea. Thus, contact lens science advanced to silicone-based contact lenses, including rigid gas permeable (RGP) contact lenses. While RGP contact lenses do afford some degree of oxygen permeability (Dk values of 30-70, with some up to 140-145), wearers continue to cite eye fatigue and eye strain as the day progresses. While wear times have increased, fatigue and strain remain issues as a result of a lack of oxygen to the cornea. Thus, what is needed is a material suitable for use as a RGP lens which exhibits ultra-high oxygen permeability (Dk value greater than 175).
Oxygen for aerobic corneal metabolism is derived principally from the atmosphere. Therefore, the physiologic integrity of the cornea during wear of a gas-permeable soft or rigid contact lens is thought to be primarily dependent on the consumption of oxygen that passes through the lens. Prediction of the physiologic performance of a contact lens on the eye, therefore, requires an index that allows the oxygen passage through the lens to be estimated.
In 1971, Fatt and St. Helen applied Fick's Law to the problem of oxygen passage through contact lenses, thereby bringing to the field the concept of oxygen transmissibility (Dk/t). This bench-top measurement has been used extensively as a basis for comparison of contact lenses. However, as Fatt has pointed out, the Dk/t term used by itself as a measure of lens performance has been “a disappointment.” The Dk/t coefficient gives a measure of the “ease” with which oxygen can diffuse through a lens; however, oxygen passage through a contact lens in a given scenario is also dependent on the driving force—that is, the partial pressure difference—across the lens. Oxygen flux (j) is the true index of the amount of oxygen that passes through a unit area of lens in a given time.